Two-dimensional images of structure of portions of the human body such as the heart are presently formed from the reflections of ultrasonic waves that are transmitted into the body.
It is also possible to determine the velocity with which blood is flowing toward and away from points in the image by using a pulsed Doppler system. An image of the velocity distribution thus attained can be superimposed on the structural image if desired.
The velocity distribution can be represented by shades of grey, but this has not been very meaningful. Furthermore, it is sometimes difficult to distinguish the velocity image from the structural image. Therefore, it has been proposed that one set of colors be respectively and arbitrarily assigned to each of a plurality of velocity ranges for blood moving in one direction, and that a different set of colors be respectively and arbitrarily assigned to each of a plurality of velocity ranges for blood moving in the opposite direction. A serious disadvantage of this method of forming a flow map is that there is no natural association of any particular color with a given velocity.
It has therefore been proposed that a red color of fixed hue be used to represent flow in one direction and that a blue color of fixed hue be used to represent flow in the other direction. Increasing velocities are represented by increasing the intensity of the appropriate color, but without a change in hue.
The problem with this type of display is that it is very difficult to perceive the difference between the various velocities because all the reds tend to look the same and all the blues tend to look the same. This is especially true for the lower levels of velocity.